1. Field of the Invention
This invention relates to the field of linear accelerators, and in particular to computer controlled radiation therapy systems.
2. Description of the Prior Art
Radiation therapy has been used extensively as a method for treating cancer patients, either alone, or in combination with surgery and chemotherapy. In typical radiation therapy systems, such as the Mevatron systems available from Siemens Medical Systems, Inc. (Iselin, N.J.), a radiation source is housed in a structure called a gantry. The apparatus includes a conventional microwave power source such as a klystron, and an accelerator structure, which may be a travelling wave or standing wave device. The accelerator produces an electron beam, which is steered through a collimating head mounted on the gantry and directed at the region to be treated. For more superficial tumors, the electron beam itself is used for treatment, because it has less impact on deeper tissue. For deeper tumors, however, high energy X rays are preferred for their penetrating power. To generate the X rays, the same electron accelerator may be used with the addition of a target made of heavy metal (e.g., gold or tantalum) placed in the electron beam path. The target emits a continuous X ray Bremsstrahlung spectrum when struck by the electron beam.
The gantry can rotate about a gantry axis which extends from the head to the foot of a treatment couch on which the patient lies, so that the radiation can enter the patient from different angles. The radiation beam coming from the accelerator is always directed through, and centered on, the gantry axis.
In applying radiation to the patient, two competing objectives are present: eliminating the malignant cells in the target region, and avoiding complications due to application of radiation to surrounding tissues. To avoid these complications, lower doses have often been applied to the targeted tumor cells than would be applied if complications were not considered, lowering the probability of successful cancer elimination. To protect surrounding tissues without compromising the treatment, it is desirable to tailor the radiation dosage to match the size, shape and location of the malignant region.
Several methods have been used in radiation therapy systems to improve control of the dosage distribution. One such method is to shape the beam profile. The "raw" beam which leaves the target has a non-uniform intensity. It is known to balance or compensate the dosage in any given space-angle rang of the radiation leaving the target by placing a compensating absorber in the beam path. U.S. Pat. No. 4,109,154 to Taumann discusses an electron accelerator in which a compensating absorber is used to shape the beam profile. The absorber absorbs overly intense radiation in the center of the beam cone.
A paper by Mantel, et al. entitled "Automatic Variation of Field Size and Dose Rate in Rotation Therapy," 2 J. Radiat. Oncol. Biol. Phys. 697 (1977) discusses a technique for changing the field size and dose rate used during rotation therapy. The gantry (and the enclosed beam forming head) rotates around the patient, so that the beam is applied from several angles. The field size and dose rate are varied as functions of the gantry angle. In this technique, the field size is adjusted in one dimension by moving a set of collimator aperture plates, or jaws, which define the beam aperture (and control the beam width), and simultaneously varying the dose rate during rotation in accordance with values selected by a computer program. The result is a more uniform dose distribution inside the target volume, and reduced dose outside that volume.
U.S. Pat. No. 4,140,129 to Heinz et al discloses a beam defining system for an electron accelerator, having an adjustable collimator and an accessory holder, to which an electron applicator is attached. The electron applicator has an wall which encloses the electron beam cone from the collimator, and an additional frame-shaped limiting aperture in order to limit the electron beam cone at the edges which face away from the beam defining system. The scattered or secondary electrons in the marginal region of the beam cone are substantially blocked by the limiting aperture. The electrons which are thus blocked have lower energy levels and, so, do not contribute to higher dosage performance deep within the patient. Thus, this device reduces undesirable irradiation of the skin surrounding the target.
U.S. Pat. Nos. 4,343,997 and 4,359,642 to Heinz, which are hereby incorporated by reference for their teachings on radiation treatment devices, describe a collimator assembly which may be used to limit or define X-ray cones of various sizes in an electron beam accelerator. A flattening filter is used with the technique to flatten the X-ray density profile. Flat dosage is achieved through the use of a collimator shielding block and one of a plurality of insert pieces or bushings which are interchangeable with one another to produce different cone angles for irradiating differently sized areas.
Another method of controlling the dosage profile is to vary the size of the beam aperture. A paper by Kijewski, et al. entitled "Wedge shaped Dose Distribution by Computer Controlled Collimator Motion" 5 Med. Phys. 426 (1978) discusses the use of a defined plate (jaw) motion to obtain a wedge-shaped isodose curve (the set of points which receive the same dose of radiation) during irradiation. FIG. 2a shows isodose profiles 30a-c achieved by this technique. The treatment begins with two collimator plates 32a, 32b separated from one another. After a predetermined time interval, plate 32a is moved towards plate 32b, which remains stationary. The movement continues until the plates meet. This causes the width of the beam 34 to become narrower as the treatment continues. The isodose curves 30a-c are deeper in the region near the stationary plate, which is exposed to radiation the longest. Such wedge shaped isodose curves may be desired in radiation therapy to adjust to anatomical conditions of the subject. A similar result may be achieved by beginning with closed plates and opening the plates. FIG. 2b shows an isodose curve in which the plates 32c, 32d begin in the closed position.
U.S. Pat. No. 5,019,713 to Schmidt discusses a radiation therapy device in which a movable aperture assembly and a non-movable filter body are combined to allow the isodose curve in the object of irradiation to rise or fall in the opening direction. At the beginning of the treatment, the plates are closed, and one plate begins to move away from the other (stationary) plate. The absorptance of the filter varies across its length. The cumulative radiation dose received at any point varies as a function of both the filter characteristics and the distance from the stationary plate, making possible non-monotonic isodose curves which vary in one dimension. For example, if the portion of the filter closest to the stationary plate has a higher absorptance, the isodose curve will have an inverted U-shape.
A paper by Levene, et al. entitled, "Computer Controlled Radiation Therapy" 129 Radiol. 769 (1978) discusses variation of dose rate, gantry angle and collimator plate position to achieve the known "arc wedge" technique.
A paper by Chin et al. entitled, "Dose Optimization with Computer Controlled Gantry Rotation, Collimator Motion and Dose Rate Variation" 9 J. Radiat. Oncol. Biol. Phys. 723 (1983) discusses a method by which continuous irradiation is simulated by summation of a large number of discrete stationary beams. Dose rate, gantry angle and collimator plate positions are varied among the beams. These methods achieve isodose contours which might not be attainable using a single stationary beam.
It is noted that the Levene et al. and Chin et al. papers relate to a conformal radiation treatment which conforms the field gradient and dose rate to a target volume using gantry rotation.
Although some of these devices allow a number of different isodose contours to be generated, the apparatus and methods used may be relatively cumbersome and time consuming.